if it was forge welded together at the same time the blade was made, it is essentially 'integral' to the blade. that would be the best. silver soldering or other low temp. soldering would also be OK.
i have heard of some less particular 'smiths' electrically welding tangs, or extensions or even wood screws to blades afterwards, which would possibly ruin any heat treatment, and weaken the join. it at least would present a discontinuity in the chemical composition of the metal where the welding rod metal was deposited...
i've also seen welded steel ships that had had great care and inspection during construction crack under stress along the weld joints.
i do not see any signs that any modern welding has taken place on this blade tho. it appears to have been entirely hand forged. final finishing probably had an assist from power tools tho, which is also OK, otherwise it would cost a lot more..
and, yes, laha to me is himalayan epoxy
tree resin, dung, brick dust, and other admixtures the kami's add to the recipe.
the inverted 'v' shape starts towards the tip from the stamped lettering forward, possibly to lighten the blade and improve the balance, it's also a bit more decorative. from above the stamped letters back to the guard it is flat. interestingly, the blade thickness swells (increases) by an additional 4mm. over about the last 4mm. forming a shoulder that the guard sets on. this joint then appears to have been hand filed and finished after welding. i see a similar flat spine transition to round or inverted V on some of my Dha's tho on those the flat extends for almost a third of the blade and is decorated with inlays.